The present invention relates to a novel internally-illuminable push-button switch unit or, more particularly, to an internally-illuminable push-button switch unit capable of being recognized even in a dark place and useful as a switching device on electronic instruments installed in mobile bodies such as car-borne audio sets, car telephones and the like as well as on electronic instruments for office and household use including pocketable calculators, telephones and the like.
Various proposals and attempts have been made heretofore for push-button switch units to be usable even in a dark place in order to ensure recognizability or visibility of the indicia appearing, for example, on the key-top. An approach therefor is to use an illuminable push-button switch unit in which the covering member for the push-button switch is provided on a light-emitting body such as LEDs. FIG. 13 of the accompanying drawing illustrates an example of such an illuminable push-button switch unit consisting generally of a base part 27, riser part 26 and key-top part 21 made from a light-transmitting material in which the key-top part 21, which supports a movable contact point of an electroconductive material on the lower surface, is provided on the top surface thereof with a light-transmitting layer 22 covered by a light-shielding colored layer 24 having an openwork 23 to indicate an indicia such as letters, signs, symbol marks and the like while a light source 25 such as an LED and the like is provided somewhere in the base part 27.
A typical method for the preparation of the covering member of the above mentioned illuminable push-button switch unit comprises the steps that a light-transmitting colored layer 22 is formed by screen printing on the top surface of the key-top part 21 made from a light-transmitting material and then a light-shielding colored covering layer 24 is formed to cover the top and side surfaces of the key-top part followed by the formation of an open-work 23 to indicate an indicia by using a laser engraver and the like. Examples of suitable material for the covering member include. silicone rubbers and organic thermoplastic elastomers such as urethane-based ones and the like in view of the requirements for the light-transmissivity to ensure visibility and elastic resilience to ensure the switching action.
The light coming from the light source 25 enters the covering member at the base part 27 and is transmitted through the covering member to reach the key-top part 21 and the light is externally emitted from the openwork 23 of indicia formed on the top surface of the light-shielding colored layer 24. Since a translucent light-diffuser layer in a suitable color, e.g., white color, is formed on the light-transmitting layer 22 in the openwork 23, the indicia given by the openwork 23 is uniformly illuminated by the light coming from the light source 25 so as to be imparted with good visibility in a dark place as well as in a well-lighted place by virtue of the contrast with the light-shielding colored layer 24 when the translucent light-diffuser layer is light-colored.
The push-button switch covering member of the above described type must satisfy several requirements: 1) that smoothness of the pushing movement of the finger tip to operate the switch should be ensured; 2) that the switching movement of the movable contact point against the fixed contact points can be felt by the pushing finger tip with reliableness; 3) that the visibility of the indicia exhibited on the top surface of the key-top can be maintained over a long period of time after many times repeated switching actions, and so on. With regard to the first requirement, the push-button switch covering members under current use are not always quite satisfactory because the top surface of the key-top made from a rubbery material such as a silicone rubber or an organic thermoplastic elastomer has its own characteristic tackiness not to ensure full smoothness of the pushing movement of the finger tip. In view of the requirement in recent years for push-button switch covering members having a more and more decreased height, it is a measure undertaken therefor to decrease the height of the riser portion 26 so that, while the feeling of reliableness in the switching operation as a consequence of the clicking touch felt by the finger tip in the buckling movement and resilience of the riser part 26, the second requirement above is satisfied less due to the decrease in the pushing stroke of the push-button switch. The third requirement for the lasting visibility of the indicia is also not always satisfied due to wearing caused by rubbing as a result of many times repeated pushing works.